The present invention generally relates to recording and reproducing of information signals and more particularly to a system for recording and reproducing an information signal on and from a recording medium.
Conventionally, information signals having a wide frequency spectrum such as video signal are recorded on a recording medium such as optical disk or magnetic tape after frequency modulation or pulse width modulation. When recording such a video signal by frequency modulation, it is well known that a sufficiently high carrier frequency at least covering a frequency range corresponding to the white level of the picture has to be used. However, there is a limitation in the frequency which can be recorded on the recording medium and thus the carrier frequency has to be limited according to such an upper frequency limit of the recording medium. Associated therewith, the frequency band of the information signal is limited and the quality of the picture is deteriorated.
In the case of recording the video signal by the pulse width modulation, an incoming video signal is converted to a pulse width modulation (PWM) signal as shown in FIG. 1(A) in which a video signal I is compared with a sawtooth wave II and a PWM signal shown in FIG. 1(B) is obtained as a result of comparison. In this technique, as the leading edge of the PWM signal is always coincident to the trailing edge of the sawtooth wave II, it is possible to record and reproduce the information signal by using a frequency-divided signal shown in FIG. 1(C) which is frequency-divided to one-half the original frequency by performing triggering responsive to the trailing edge of the PWM signal of FIG. 1(B). In this case, one can record the information signal without problem even when the frequency of the sawtooth wave shown in FIG. 1(A) is doubled and the frequency spectrum of the video signal recorded on the recording medium can be increased substantially.
When recording the PWM signal on an optical disk, a laser beam is used for the recording such that the intensity of the laser beam is changed according to the frequency-divided signal shown in FIG. 1(C) and a row of pits are formed on the optical disk as shown in FIG. 1(D) in correspondence to the signal of FIG. 1(C).
When recording the PWM signal in a form of pits shown in FIG. 1(D), there arises a problem in that a width d.sub.1 of the pit shown in FIG. 1(C) tends to be changed apparatus by apparatus as shown in FIGS. 1(D) and (E) when the beam spot of the laser beam used for recording is not controlled exactly at a uniform spot radius. In the case of FIG. 1(D), the pit width is reduced to d.sub.2 which is smaller than d.sub.1 while in the case of FIG. 1(E), the pit width is increased to d.sub.3 which is larger than d.sub.1. Note that in any of these cases, the width of the pit does not correspond to the PWM signal of FIG. 1(C). A similar problem occurs also at the time the information signal is reproduced when the beam spot of the laser beam used for reproducing is not controlled exactly. When such a variation of the beam spot occurs at the time of recording or reproducing, therefore, pulse widths T.sub.0, T.sub.1, T.sub.2, . . . of the original PWM signal shown in FIG. 1(B) cannot be reproduced from the frequency-divided PWM signal of FIG. 1(C). In other words, the recording and reproducing of the wide spectrum video signal becomes no longer possible.
A same problem occurs also when the frequency-divided PWM signal is recorded on a magnetic tape as a result of variation of the state of contact between the magnetic head and the magnetic tape.